1. Field of the Invention
The invention concerns an optical fiber that propagates only one polarization state of the fundamental mode and so can be used to polarize light or to propagate polarized light over long distances.
2. Description of the Related Art
At the present time, when it is necessary to polarize light for transmission through single-mode optical fibers, this is usually done with either a bulk optic polarizer or an integrated optical polarizer, both of which are quite expensive and undesirably large for many optical fiber applications.
In the mid 1980's, York Technologies introduced a polarizer based on a single-mode, single-polarization optical fiber that has been tightly coiled and supplied in a sealed housing. The coiling results in a bending loss which, in combination with stress-induced birefringence, causes one polarization to be attenuated while the orthogonal state propagates down the fiber core. Because the York device has a rather narrow bandwidth, it must be matched with any single-mode optical fiber with which it is used. A York brochure marked 6/87 (apparently June 1987) reports a bandwidth of at least 20 nm. A publication by the developers of the York device indicates that the optical fiber of the York polarizer has an asymmetric stress-applying region of a cross-sectional geometry resembling a bow-tie and "a depressed index giving a W index profile in one direction", i.e., an index of refraction profile in the shape of a W. See Varnham et al.: "Single Polarization Operation of Highly Birefringent Bow-Tie Optical Fibres," Elec. Lett., Vol. 19, pp. 679-680 (1983).
FIGS. 1, 3, and 5 of U.S. Pat. No. 4,515,436 (Howard et al.) showing three single-mode, single-polarization optical fibers that also have a depressed index or W index profile, and more specifically "the refractive index of an outer cladding region is greater than the refractive index of an inner cladding region but less than that of the core region" (Abstract). Operation at an intermediate wavelength allows the fiber to act as a polarizer. Howard says in connection with FIGS. 1 and 2:
"The birefringence decreases with distance from core 10 so that maximum bandwidth requires a large inner cladding 12. However, this inner cladding region 12 should be narrow for rapid tunneling loss of the undesired polarization. Therefore, a trade-off exists between providing rapid tunneling loss and providing a large bandwidth" (col. 4, lines 18-25). PA1 "A single-polarization fiber which uses a combination of high stress-induced birefringence combined with a depressed or W-type cladding structure. . . . The depressed cladding provides a tunneling loss which increases rapidly with wavelength. The anisotropic stress created by a highly-doped elliptical cladding splits the mode-effective indexes so that the cutoff wavelength differs for the two polarizations. Bandwidths of 8 percent are achieved for fibers with core sizes and refractive indexes typical of single-mode transmission fibers. Extinction ratios of more than 30 dB with less than 1-dB insertion loss have been obtained with fiber lengths on the order of 1 m. The wavelength of useful operation can be tuned by bending the fiber" (page 370).
Although the Howard optical fiber can act as a polarizer without being coiled or otherwise bent, the embodiment of FIG. 5 was bent into four turns with a 1.5 cm bending radius to obtain the transmission data of FIG. 6 which indicates a bandwidth of about 25 nm at 570 nm.
Simpson et al.: "A Single-Polarization Fiber, J. of Lightwave Tech, Vol. LT-1, No. 2 (1983) describes
See also Simpson et al.: "Properties of Rectangular Polarizing and Polarization Maintaining Fiber," Proc. SPIE, Vol. 719, pp. 220-225 (1986) which says that a polarizing fiber of substantially rectangular shape with a W-type index profile had provided greater than 30 dB of extinction ratio at a length of 5 cm, with a modal birefringence which separates the orthogonal polarization mode cutoff wavelengths by 0.1 um.
A "W-tunneling fiber polarizer" is held straight for testing in Stolen et al.: "Short W-Tunneling Fibre Polarizers," Elec. Lett., Vol. 24, pp. 524-525 (1988) which indicates a bandwidth of about 25 nm at 633 nm to achieve 39 dB polarization. Unfortunately, the bandwidth of about 25 nm of Stolen and some of the other above-discussed publications, leaves little margin for manufacturing error. For example, it is difficult to build a narrow-bandwidth semiconductor light source (e.g., a laser) to a precise operating wavelength. Also, shifts in bandwidths could be expected in a series of optical fibers made to duplicate the Stolen or other prior optical fiber polarizers. Furthermore, variations in conditions of use could result in occasional mismatching of wavelengths even if a laser and a polarizer had been initially matched.
Okamoto et al.: "High-Birefringence Polarizing Fiber with Flat Cladding," J. of Lightwave Tech., Vol. LT-3, No. 4, pp. 758-762 (1985) remarks that the Varnham publication shows "that the polarizing effect can be enhanced when the birefringent fiber is bent with the fast axis oriented parallel to the plane of the fiber coil." Okamoto's fiber, which has a depressed or W index/profile along the x-axis (slow axis) has flats that are parallel to its slow axis. When the fiber is coiled, the flats keep the slow axis parallel, and this produces substantially broader bandwidths. FIG. 9 of Okamoto shows that a bandwidth of 390 nm can be attained at a bending diameter of 4.5 cm. See also Okamoto et al.: "Single-Polarization Operation of Highly Birefringent Optical Fibres," Applied Optics, Vol. 23, No. 15, pp. 2638-2641 (1984) and U.S. Pat. No. 4,480,897 (Okamoto et al.).